1. Field of the Invention
This invention relates to a color image forming process. More particularly, the invention relates to a color image forming process which comprises processing a photographic element containing silver image-wise distributed therein in the presence of a complexing agent, an oxidizing agent and a dye or dyes, to oxidatively bleach the dye or dyes.
2. Description of the Prior Art
In a general process of forming color images, azomethine dyes or indoaniline dyes are formed by developing silver halide light-sensitive material in the presence of couplers using a primary aromatic amine developing agent. The color development system using silver halide is based on the process invented by L. D. Mannes and L. Godowosky in 1935. Various improvements have been made in the process since then, and the system has usually been employed worldwide in the photographic art.
The color development system using a primary aromatic amine developing agent generally has the following disadvantages. That is, (1) the dyes formed by the system have poor light resistance, heat resistance, and moisture resistance and, hence, the color images formed show a great tendency toward fading with the passage of time, (2) a primary aromatic amine developing agent is toxic to the human body, for example, causing a poisoning of the skin and, thus, specific precautions are required in using this type of developing agent, and (3) since there is an equivalent relationship between the dye image and the oxidation product of the color developing agent, it is theoretically difficult to reduce the amount of the silver halide which takes part in the dye formation to an amount lower than the stoichiometrically required amount.
Conventional techniques of reducing the amount of silver halide in color photography can be classified into reducing the amount of silver halide present over the stoichiometrically required amount thereof as low as possible and reducing the stoichiometrically required amount of silver halide itself. In regard to the latter case, the so-called two equivalent couplers capable of forming one molecule of dye with two molecules of silver halide have been developed. However, even using this technique thus developed, it is theoretically difficult to reduce the amount of silver salt in the light-sensitive materials to less than 1/2 of the amount of silver salt in light-sensitive materials containing couplers other than two equivalent couplers.
A color photographic process other than the above-mentioned processes employed at present is based on a silver-dye-bleach photographic process. This process is also based on the color photographic process disclosed in U.S. Pat. No. 2,270,118 and, since azo dyes are used in the color process, the color images formed by the process generally have excellent light resistance, heat resistance and moisture resistance.
A typical photographic element used for the silver-dye-bleach color photographic has three silver halide photographic emulsion layers respectively sensitized to red, green, and blue light, and having associated therewith, respectively, a bleachable cyan, magenta and yellow dye. Such a photographic element provides color photographic positive images through the following processing:
(1) The photographic element is image-wise exposed. PA1 (2) The exposed photographic element is developed in a silver halide developer to form negative silver images, the photographic element is then processed in a dye bleach bath which oxidizes the silver images to a silver salt and concurrently decolorizes the associated dye pattern, and, finally the photographic element is fixed and washed to remove the residual silver salt, whereby dye images are obtained which are photographically the reverse of the initial silver images. The silver-dye-bleach process is generally described in, for example, U.S. Pat. Nos. 3,498,787 and 3,503,741, Canadian Pat. No. 790,533 and A. Meyer, "Some Features of the Silver-Dye Bleach Process", The Journal of Photographic Science, Vol. 13, 90-97 (1965). PA1 (1) Since a large amount of silver is required for bleaching the dyes, the photosensitive materials must contain a large amount of silver halide in the silver halide photographic emulsion layers. PA1 (2) Since a strongly acidic processing solution which is highly corrosive is usually used in these processes, difficulties are encountered in preserving and handling the solution. PA1 [E.degree. of the oxidant is within the range of +0.15 to +0.8 volt, preferably +0.4 to +0.8 volt] PA1 (Meier, column 2 lines 8 to 12) PA1 (Meier, column 2 lines 34 to 36) PA1 (Meier, column 3 line 41)
In the silver dye bleach process as described in U.S. Pat. No. 2,270,118, dye images are formed by processing dye-containing layers having silver images with an acid solution which decomposes the dyes at the silver-containing areas. The decomposition or destruction of the dye is accelerated by various "catalysts" such as phenazine. Also, the reaction in these dye bleach systems is considered to proceed on a stoichiometric basis (for example, it is suggested that 4 atoms of silver are required for decomposing one azo dye group in Column 1, lines 18-21 of U.S. Pat. No. 3,340,060).
However, these silver dye bleach processes have the following disadvantages:
Recently, numerous investigations have been made for saving silver as a resource, increasing the efficiency of the reaction system, and improving the quality of the color images formed by reducing the amount of silver required to decompose each molecule of dye.
Several patents are known which deal with various types of image forming processes, for example, U.S. Pat. No. 3,716,362 Meier, U.S. Pat. No. 3,259,497 Wartburg and U.S. Pat. No. 2,564,238 Sprung. These patents are discussed below and compared to the present invention, disclosure relative to the present invention not being part of the prior art, of course, but being offered to offer a valid comparison to the prior art. Reference should also be made to later discussed FIG. 2 for a complete understanding of the subject matter involved.
Turning to U.S. Pat. No. 3,716,362, this patent teaches a process at column 1 lines 64 to 65 wherein metallic silver is removed from a photographic maerial without decomposition of a dye. That is, the reaction of Ag.degree..fwdarw.Ag.sup.+ L.sub.2 occurs in this process as shown in FIG. 2. Further, the reaction of Meier involves materials which exhibit the following relationship between their complex forming constant and oxidation-reduction potential: ##EQU1## where R, F and T denote gas constant, Faraday constant and temperature respectively.
When a complexing agent is present, E.degree.(Ag.sup.+ .fwdarw.Ag.degree.) is constant and, therefore, the oxidation-reduction potential of Ag can be represented by the complex forming constant K.sub.1.
On the other hand, the present invention teaches a process in which a dye is decomposed at the areas where silver is present, i.e., metallic silver is oxidized by a peroxo sulfate into the Ag(I) complex and the Ag(I) complex is further oxidized by the peroxo sulfate into the Ag(II) complex. In the reaction the complexing agent acts to reduce the oxidation-reduction potential of silver (i.e., Ag.sup..sym. .fwdarw.Ag.sup.2.sym.); thus, this process can be schematically illustrated as follows: ##EQU2## where K.sub.2 represents the complex forming constant with Ag.sup.2.sym., and K.sub.1 represents the complex forming constant with Ag.sup..sym.. As is apparent from the above schematic, the complex forming constants directly influence the reaction in the form K.sub.1 /K.sub.2 and, therefore, E.degree. cannot be expressed by K.sub.1 as in Meier. Thus, in the present invention E.degree.(Ag.sup.2.sym. L.sub.2 +e.sup..crclbar. .revreaction.Ag.sup..sym. L.sub.2) is used to define the complexing agent.
The process of this invention differs from the process of Meier as follows. ##STR1##